1. Field of the Invention
The present invention relates to reduction of vortex-induced vibrations on submerged cylindrical pipelines, risers, and tendons. Specifically, this invention relates to the vortex induced vibration inhibitors installed on submerged cylindrical pipelines, risers, and tendons.
2. Description of the Related Art
Submerged cylindrical pipelines, risers, and tendons are subjected to vibrations caused by the periodic shedding of eddies resulting from fluid flow. The vibrations place additional stress on the pipelines, risers, and tendons and reduce their fatigue lives. Vortex induced vibration inhibitors (VIVIs) are installed onto the vibration sensitive components to eliminate the effects of vortex-induced vibrations.
Strakes are cylindrically shaped housings with fixed fins helically attached to the outer surface. Typically, but not always, strakes are installed on risers, pipelines, and tendons along the sections close to the water's surface where both current and wave forces are encountered.
Fairings have a streamlined (sometimes tear-dropped) cross-sectional shape and include a bearing surface so that they rotate around the riser, pipeline or tendon with the direction of current flow. Typically, fairings are used at depths well below the surface so that wave forces do not cause premature wear of the rotational bearing pads.
Many deepwater offshore platforms and many floating platforms have been installed without VIVIs. It was learned that ocean currents induced vibrations in platform support structures. The solution to eliminating vortex-induced vibrations on existing structures is to add fixed-fin vortex strakes or fairings to the long cylindrical components of these structures. In light of more recent findings showing that ocean currents are much stronger than originally thought, retrofitting existing structures with VIVIs is desirable.
The preferred method of retrofitting existing risers, pipelines, and tendons with VIVIs is to utilize a remotely operable vehicle (ROV). An ROV is an underwater robot that is typically controlled from the surface. The ROV may be equipped with hydraulic manipulators, or arms, to assist in performing subsea tasks, and with special tooling to install VIVIs onto underwater structures.
To prepare prior art VIVIs for installation by an ROV, the VIVIs are retrofitted with hardware, such as handles and ropes. Several retrofitted VIVIs may be placed onto a piece of staging equipment that is lowered to an area proximate the submerged component or the retrofitted VIVI may be lowered individually by a crane. An ROV, equipped with arms, grasps one VIVI at a time and transports it to a submerged cylindrical component.
A vision system and a remote control system allow an operator to maneuver the ROV to the staging area and remove the VIVI. Once the submerged component is nested within the VIVI, the ROV disengages from the VIVI and maneuvers to the opposite side of the VIVI and component. The ROV operator then uses the vision system to locate handles or ropes and grasp them with tooling on the ROV arms. The arms may then be moved to close the VIVI around the submerged component.
Prior art VIVIs are often destroyed when they are forceably removed. Further, the VIVI often damages the coating on the submerged components as they are removed. It would be an improvement to the art to have a releasable locking mechanism that permits the removal of a VIVI without damaging either the VIVI or the coating of the submerged component.
It would be an improvement to the art to have an engagement and locking system on a VIVI that reduced the amount of time an ROV requires to install a VIVI.
It would be a further improvement to the art to have engagement points that permit the use of simple tooling on the ROV to engage and install the VIVIs around a submerged component.
It would be a further improvement of the art to have a system where multiple VIVI's may be efficiently installed around a submerged component.